There is no known industrially available machine today for recycling graphite dust and fines. Some have used pharmaceutical equipment for making pills wherein the powder is pressed but then must be baked, which adds cost. Converting the dust into a pellet or sphere would be attractive, because when graphite powder is used for steel making and other graphite processing, particles become airborne causing material loss and industrial hygiene concerns.
Typical carbon plants can make about 15 million pounds of graphite a year, and generate about 1 to 1.5 million pounds of dust. There is a production cost advantage to bagging the dust and fines and selling them as waste. Some of the dust can have very high purity (e.g. >=99% pure) particles, thus suitable for being reground and screened to make particles of the size desired. The dust and fines are not reprocessed but are collected and processed as waste.
Carbon spheres having a particle diameter smaller than 1 micron can be produced by techniques utilized for the production of carbon black such as furnace black or channel black. It is much more difficult to obtain carbon spheres having a particle diameter in the range of from 1 to 20 microns. These carbon spheres generally tend to exhibit a phenomenon wherein several adjacent carbon spheres adhere to form a fused cluster. Methods for the production of carbon spheres or hollow carbon spheres resistant to mutual cohesion have been disclosed in the specification of U.S. Pat. No. 3,786,134. This method relates to the manufacture of carbon spheres having particle diameters not smaller than 30 microns and is a process using virgin materials.
Another source of graphite waste is kish, which refers generally to carbon material that collects at the surface of molten iron (hot metal) from a blast furnace after casting. At casting temperatures, the liquid iron is saturated with dissolved carbon. As the iron cools, it becomes supersaturated with carbon. The carbon comes out of the iron solution as flakes of graphite. Graphite is a soft mineral that occurs as thin plates and is composed of pure carbon. The longer the hot metal cools, the greater the yield of graphite flakes.
In BOF (basic oxygen furnace) steelmaking operations, kish is also produced during the steps of reladling, desulphurization of the hot metal, slag skimming, and ladle treatment. Kish produced during these steps is collected as baghouse dust and varies in quantity and composition. The quantity of kish materials collected as baghouse dust is significant. Disposal of the kish, recycling or other treatment, requires some handling. The kish dust is difficult to handle. The kish dust is dry and contains an ultra fine component.
The distribution of particle sizes of the kish dust shows that 30 to 40 percent of the kish has a particle size less than 25 microns. High-carbon dust with a particle size greater than 10 microns exhibits a tendency to settle to the ground in ambient air. Part of the kish, however, has a particle size smaller than 10 microns. Particle sizes between about 1 and 10 microns exhibit a resistance to settling both in ambient air and water that increases as the particle size decreases. Below a particle size of about 1 micron, a significant amount of the kish dust remains airborne and does not settle. Thus, part of the kish dust is so fine that it tends to remain airborne. This makes it easy for wind to carry the kish dust far from the source of kish, especially during handling where the dust may become agitated during handling and disposal. An area surrounding the source of kish therefore becomes susceptible to contamination, especially if attempts are made to handle or transport the kish for recycling, beneficiation, and/or disposal.
The tendency for the kish to contaminate surrounding areas is exacerbated by its physical properties. The graphite contained in the kish dust exhibits hydrophobic properties. The hydrophobic properties inhibit wetting. When attempts are made to contain the dust by applying water to the dry kish, the kish dust floats on the water. Water application therefore falls well short of alleviating the dustiness and difficulties associated with handling of the kish. It also falls well short of alleviating the problem of environmental contamination.
In view of the hydrophobic properties of the graphite, attempts have been made to apply a surface active agent (i.e., a surfactant) to the kish, in order to provide a wetting agent along with the water as taught in U.S. Pat. No. 3,932,596 to Rohatgi. While those attempts were successful to some extent at containing the kish temporarily, eventually the resulting combination of kish, water, and surfactant would dry. When it dried, the kish again became dusty, and was able to contaminate the surrounding area.
The potential for contamination, however, is not the only disadvantage of the prior techniques. The difficulty associated with handling of the kish and its tendency to become airborne during handling has a negative impact on the recovery of commercially valuable material, such as graphite, from the kish. In particular, the loss of kish dust into the air reduces the total amount of material that can be recovered from the kish.
There is consequently a need in the art for a way of agglomerating graphite such as kish, so that it can be handled and/or stored with little, if any, of the kish contaminating or becoming suspended in the surrounding air. A need also exists for a graphite product that can be transported easily for recycling, beneficiation, and/or disposal, with little or no contamination of the surrounding air, and which can be handled and/or treated to recover such materials.